Process and apparatus for cracking hydrocarbons

ABSTRACT

The cracking of hydrocarbon fractions, for example to obtain low olefins, in particular ethylene, is performed in reaction tubes that are at least partially coated with catalyst. The catalyst can promote gasification of coke with water vapor after the water-gas reaction in CO, CO 2  and H 2 , and/or the cracking reactions. The catalyst coating can be placed directly on reaction tubes or on top of a previously applied adhesion promoter and/or auxiliary medium placed on the reaction tubes, in particular on a grid. Preferably, the catalyst coating is applied thermally, by cold-coating processes, in particular the slip process, by vapor deposition and/or,adhesion. The catalyst can promote, in particular, gasification of coke with water vapor after the water-gas reaction in CO, CO 2  and H 2  and make hydrogen, obtained during catalytic gasification, available for cracking hydrocarbons and/or as additional product. As catalyst, one based on calcium aluminate is especially suitable, preferably with a doping of alkali vanadate, in particular of potassium pyrovanadate. Cracking can be performed by, for example, steam cracking, pyrolysis, thermocatalytic cracking or steam reforming.

SUMMARY OF THE INVENTION

[0001] The invention relates to a process for cracking hydrocarbons, hydrocarbon mixtures or hydrocarbon fractions and an apparatus having a catalyst coating.

[0002] Cracking processes can be used to achieve various objectives. Steam cracking, for example, is usually used to obtain low olefins, in particular ethylene. As feed material for ethylene producing steam cracking units, hydrocarbon mixtures containing ethane such as gas oil can be used. Other known processes for cracking hydrocarbons, such as steam reforming, are used, as a rule, in connection with production of synthesis gas.

[0003] During thermal cracking of hydrocarbons, in particular heavy hydrocarbons, cracking furnaces with at least one convection zone and at least one radiant zone are used, as is known. In such a case, thermal cracking is performed in a burner-heated radiant zone while, in the convection zone, hydrocarbons and other fluids are heated against flue gas by heat exchangers placed therein. Usually, hydrocarbons and process vapor are preheated in the heat exchangers of the convection zone and a hydrocarbon/vapor mixture is fed to the coils placed in the radiant zone. The product gases are quickly cooled in a downstream quench exchanger to interrupt the reactions. Such processes and cracking furnaces are known)for example from DE-OS 28 30 824 and DE-PS 28 54 061 and U.S. Pat. No. 4,361,478. See also Linde Reports on Science and Technology, 48, pp. 9-10 (1991).

[0004] During cracking processes, coke precipitates on the various parts and devices of the cracking unit, for example, within the cracking tubes as well as within the quench exchangers. This leads, for example in conventional steam crackers, to the fact that decoking must be performed usually at regular intervals. Decoking causes production downtimes that must be compensated for by spare furnaces. This applies in principle to all hydrocarbon feed materials. Under equal cracking activity and retention time, the heavier the hydrocarbon feed used, the shorter the intervals between two decokings become and thus total downtimes increase. The decokings furthermore require additional maintenance expense.

[0005] From patent DD-243 708 A1 is known a process for thermocatalytic cracking of higher-boiling hydrocarbons. Here, a gasification catalyst is used that is based on calcium aluminate and is doped with an alkali vanadate such as potassium pyrovanadate. Such a gasification catalyst and a process for its production are described in German patent DD-243 647 A1.

[0006] In processes for cracking hydrocarbons in conventional cracking tubes or coils, the cracking tubes or coils can be filled, for example for thermocatalytic cracking, at least partially with a gasification catalyst. It has been shown that catalyst fillings in conventional reaction tubes or coils of pyrolysis furnaces or steam crackers entails a series of drawbacks. Because of the catalyst fillings within the reaction tubes or coils, the pressure loss of the individual reaction tubes/coils increases considerably. Further, the inherent weight of the reaction tubes/coils filled with catalyst is clearly higher in comparison to unfilled reaction tubes/coils, thereby increasing the mechanical stress. To compensate for the effect of large pressure drops in the reaction tubes or coils, the wall thickness of the reaction tubes/coils must be increased, which has a negative effect on heat transfer. Further, because of the volume of the catalyst, the space-time yield of the reaction tubes/coils is reduced so that new furnace concepts are needed. Patent DE 44 00 430 A1 describes a process for thermocatalytic cracking of higher-boiling hydrocarbons and a cracking furnace, the problems occurring because of the catalyst fillings are taken into account by the use of straight reaction tubes without elbows.

[0007] An object of the invention is to provide a process and an apparatus of the above-named types that avoid the named drawbacks and/or improve cracking processes overall and can optionally simplify them. In particular, it is possible to perform cracking without, or with only limited, losses of operating time due to downtimes caused by decoking, even at high levels of cracking activity. Further, problems occurring because of catalyst fillings in the cracking tubes are avoided as much as possible.

[0008] Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.

[0009] These objects are achieved for the process according to the invention by performing cracking within reaction tubes that are coated on their inside, at least partially, with catalyst.

[0010] The catalyst coating can be applied directly onto the internal surface of the reaction tubes. Alternatively, the catalyst coating can be applied on an adhesion promoter layer previously applied to the internal surface of the reaction tubes. Further, the catalyst coating can be applied onto an auxiliary medium, either directly or on top of an adhesion promotion layer. The auxiliary medium such as, for example, a tubular shaped grid, is then placed within the reaction tube adjacent the internal surface thereof. These types of catalyst coatings can be used in combination, e.g., the straight parts of a reaction tube can be coated directly with the catalyst or with layers of adhesion promoter and catalyst and bent parts of the reaction tubes can be provided with a catalyst coated auxiliary medium (which is much easier to apply in a bent tube section than is direct coating). The catalyst coatings according to the invention make it possible, in principle, to use all known shapes of reaction tubes for the cracking. Further, the catalyst coatings do not lead to considerable pressure loss occurring in the reaction tubes. Because the space/time yield is not changed by the coating, the process is especially attractive for use in existing units.

[0011] The catalyst applied as a coating in accordance with the invention can promote the gasification of coke with water vapor, via the water-gas reaction to produce CO, CO₂ and H₂, and/or promote the cracking reactions. Water vapor can be introduced with the hydrocarbon feed material. Usually, however, process steam is admixed with the hydrocarbon feedstream after preheating the feed and superheating the process steam. The endothermic water-gas reaction occurs according to the reaction equations

C+H₂O

CO+H₂  (1)

[0012] and

CO+H₂O

CO₂+H₂  (2).

[0013] Among other things, because of the fact that hydrogen is produced and thus made available, the cracking of hydrocarbons into light hydrocarbons is supported. But a suitably selected catalyst can also promote the cracking reaction directly.

[0014] The catalyst coating according to the invention can be applied by any known application process, preferably by thermal processes, by cold-coating processes such as, in particular, the slip process, by vapor deposition processes and/or by adhesion processes.

[0015] Suitable thermal coating processes encompass, in particular, flame spraying, flame burst spraying, high-speed flame spraying, plasma spraying, arc spraying and molten bath spraying.

[0016] In the slip process, catalyst powder is placed in an aqueous solution and laced with a bonding agent. The portion of bonding agent is generally below 10 wt. %, for example, about 0.1-10 wt. %, preferably 1-10 wt. %. Finally, the solution is applied as a suspension, for example by dipping, spraying, brushing, or the like. The layer, applied in this manner, is generally still subsequently treated thermally, for example, dried, baked or sintered. As bonding agents, phosphates, silicates and/or aluminates are particularly suitable. Such a slip coating can also be applied on a thermally sprayed (for example, by arc spraying) adhesion-promoting layer.

[0017] Among the suitable vapor deposition processes are the processes known as PVD-processes (physical vapor deposition) and as CVD-processes (chemical vapor deposition) As already described, the catalyst can promote gasification of coke with water vapor after the watergas reaction in CO, CO₂ and H₂. The hydrogen obtained during the catalytic gasification can be used for cracking the hydrocarbons. In addition, the hydrogen obtained can also advantageously be available as an additional product, as is sought, for example, in production of synthesis gas.

[0018] In configuring the invention, a catalyst based on calcium aluminate, preferably with a doping of alkali vanadate, in particular potassium pyrovanadate, is used, because, on the one hand, it is excellently suited for the water-gas reaction and, on the other hand, can be applied by the application processes described above. Calcium aluminate catalysts can promote both the coke gasification and the hydrocarbon cracking. The invention can employ any of the known catalysts for coke gasification and/or hydrocarbon cracking. Suitable gasification catalysts are described, for example, in DD-243 708 A1, DD-243 647 A1 or patent application DE 44 00 430 A1. Catalysts comprising CaO, MgO and/or calcinated dolomites known from “Thermal and Catalytic Cracking of n-Heptane in Presence of CaO, MgO, and Calcinated Dolomites,” G. Taralas, V. Vassilatos, K. Sjoestroem and J. Delgado, The Canadian Journal of Chemical Engineering, Volume 69, December 1991, pages 1413-1419, are also suitable for catalyzing the coke gasification or the hydrocarbon cracking. The gasification activity of the catalyst is determined by its composition. By selection of the gasification catalyst composition, the gasification activity of the catalyst can be matched to the coking tendency of the hydrocarbon feed material. Thus, excessive gasification of the cracking feed material can be prevented. In any case, the gasification activity of the catalyst should be sufficient to prevent carbon and/or coke precipitation in the reaction tubes, or to limit it to an extent that does no damage. Experiments have surprisingly shown that, for example, during thermocatalytic cracking, a high gasification activity does not lead to a reduction in yield, but rather, with high gasification activity, the amount of pyrolysis oil, i.e., the product fraction with a boiling point above about 200° C., clearly decreases.

[0019] Particular advantages are to be achieved with the process according to the invention when the cracking involves vapor cracking, pyrolysis, thermocatalytic cracking or steam reforming. It has been shown that, with the use of the coated reaction tubes, for example in steam crackers or during thermocatalytic cracking, the olefin yield of the cracking product turns out to be especially high with a short residence time, and good selectivity of the cracking is achieved.

[0020] The quality of the feed material of the hydrocarbon fractions is usually characterized by the BMCI value (Bureau of Mines Correlation Index), which roughly corresponds to the aromatic content of the hydrocarbon feed material. Feed materials with a relatively low BMCI value can thus be especially well cracked. With the thermal cracking process according to the invention, hydrocarbon feed materials up to a BMCI value of about 60 can be economically processed. Generally, the BMCI value of the hydrocarbon feed-stocks is about 20-65, preferably 25-60, especially preferably 40-55. Additional advantages of the process according to the invention result when the mass speed has a value of preferably about 5-300 kg/s·m², especially 10-200 kg/s·m², especially preferably 20-120 kg/s·m². Mass speed is the ratio of mass flow rate (i.e., the sum of the mass flow rate of hydrocarbon feed material and mass flow rate of vapor (kg/s)) to cross sectional area of the tube (m²).

[0021] In configuring the process according to the invention, the average internal surface load of the reaction tubes preferably remains at a value of about 10-120 kW/m², especially 15-80 kW/m². With such a heating surface load imposed on the reaction tubes, large heat transfer or large heat input into the hydrocarbon/vapor mixture in the reaction tubes is ensured, without unduly stressing the reaction tubes.

[0022] Advantageously, the product gasses can be quickly cooled in tubes of a cracked gas cooler or quench exchanger. The tubes of the cracked gas cooler or quench exchanger and/or other surfaces therein can be provided with a catalyst coating as described above. Here, also in the cracked gas cooler or quench exchanger, e.g., TLE or TLX (transfer line exchanger), coking is prevented or considerably limited.

[0023] The catalyst coating according to the invention can be used in a simple way for retrofitting existing cracking units. Existing cracking units can be effectively improved by subsequently applying the catalyst coating.

[0024] The invention further relates to a cracking furnace for cracking hydrocarbons or hydrocarbon mixtures in reaction tubes, whose reaction tubes are coated with a catalyst according to the invention.

[0025] The invention further relates to a steam reformer with reaction tubes, whose reaction tubes are coated with a catalyst according to the invention.

[0026] The invention finally relates to an apparatus, in particular a cracking furnace, a steam reformer, a reactor, a quench exchanger, an evaporator and/or a heat exchanger having surfaces that come into contact with carbon and/or carbon compounds, in particular hydrocarbons, during operation thereof, wherein these surfaces, in particular tube and/or wall surfaces, are coated at least partially with a catalyst in accordance with the invention, to reduce or avoid coke deposits and to promote gasification of coke and/or to promote cracking reactions. Such an apparatus exhibits, because of the now eliminated or at least considerably reduced production downtimes, increased unit capacity. In addition, because of the invention, the necessary maintenance expense for the apparatus can be reduced.

[0027] There is a simple way of installing the coating on reaction tubes and/or surfaces. This applies for the construction of new units and for retrofitting of existing units. The coating can be applied directly to the reaction tubes or surfaces or on top of an adhesion promoter previously applied to the reaction tube or surfaces and/or applied onto an auxiliary medium placed within the reaction tubes or on the surfaces. In particular, the auxiliary medium can be a grid that is preferably preshaped in the form of a tube. The auxiliary media here are positioned in the reaction tubes adjacent their internal surfaces. The coating is applied especially easily and lastingly, when the surfaces, reaction tubes and/or auxiliary media are coated by a thermal coating process, a cold coating process (particularly a slip coating process) a vapor-deposition coating process and/or an adhesive coating process. Optionally the catalyst coating is applied to an adhesion-promoting layer.

[0028] The catalyst coating can be made from any catalyst that is known to promote the water-gas reaction and/or the cracking reaction. Catalysts comprising CaO, MgO, and/or calcinated dolomites, which promote the watergas reaction, are known, for example, from “Thermal and Catalytic Cracking of n-Heptane in Presence of CaO, MgO, and Calcinated Dolomites,” G. Taralas, V. Vassilatos, K. Sjoestroem and J. Delgado, The Canadian Journal of Chemical Engineering, Volume 69, December 1991, pages 1413 to 1419. Especially suitable as a catalyst coating is the above-described coating made of a catalyst based on calcium aluminate, preferably with a doping of alkali vanadate, particularly potassium pyrovanadate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:

[0030]FIG. 1 illustrates a cracking furnace with a convection zone and radiant zone; and

[0031]FIG. 2 illustrates coke gasification in a reaction tube in accordance with the invention.

DETAILED DESCRIPTION

[0032]FIG. 1 illustrates a typical cracking furnace showing a convection zone, radiant zone, quench exchanger and steam drum. In accordance with the invention, the reaction tube is a catalyst coated reaction coil. A cracking or radiant coil has generally the shape of a meandering tube. See “Modern Furnace Design for Steam Crackers,” FIG. 2, page 10, which shows some typical examples of such coils.

[0033]FIG. 2 illustrates coke gasification at the surface of a reaction tube coated with catalyst in accordance with the invention. The coke is gasified with water vapor in the presence of the catalyst in accordance with equation (1) to form CO and H₂. In accordance with equation (2), the CO reacts with water vapor to form CO₂ and H₂.

[0034] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0035] In the foregoing, all temperatures are set forth uncorrected in degrees Celsius and unless otherwise indicated, all parts and percentages are by weight.

[0036] The entire disclosure of all applications, patents and publications, cited above, and of corresponding German applications P 195 05 455.5, filed Feb. 17, 1995, and P 196 00 684.8, filed Jan. 10, 1996, are hereby incorporated by reference.

[0037] The preceding can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used therein.

[0038] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

What is claimed is:
 1. In a process for cracking hydrocarbons, hydrocarbon mixtures or hydrocarbon fractions, the improvement wherein cracking is performed in reaction tubes coated at least partially with catalyst.
 2. A process according to claim 1, wherein said catalyst promotes gasification of coke with water vapor after the water-gas reaction in CO, CO₂, and H₂, promotes cracking reactions; or promotes both cracking reactions and gasification of coke with water vapor.
 3. A process according to claim 1, wherein the catalyst coating is applied directly to said reaction tubes, is applied to reaction tubes to which an adhesion promoter has been previously applied, and/or is applied to an auxiliary medium placed in said reaction tubes.
 4. A process according to claim 1, wherein said catalyst is applied to auxiliary mediums in the form of tubular grids which are positioned within said reaction tubes.
 5. A process according to claim 1, wherein the catalyst coating is applied by a thermal process, by a cold coating process, by a vapor deposition process or by an adhesion process.
 6. A process according to claim 2, wherein the catalyst promotes gasification of coke with water vapor after the water-gas reaction in CO, CO₂ and H₂, whereby hydrogen obtained during gasification is available for cracking of hydrocarbons; as an additional product, or both.
 7. A process according to claim 1, wherein said catalyst contains calcium aluminate.
 8. A process according to claim 7, wherein said catalyst is doped with alkali vanadate.
 9. A process according to claim 8, wherein said alkali vanadate is potassium pyrovanadate.
 10. A process according to claim 1, wherein cracking is performed by vapor cracking, pyrolysis, thermocatalytic cracking or steam reforming.
 11. A process according to claim 1, wherein the mass speed is 5-300 kg/s·m².
 12. A process according to claim 1, wherein the average internal sur face load on said reaction tube,is 10-120 kW/m².
 13. A process according to claim 1, wherein product gasses are quickly cooled in a cracked gas cooler or quench exchanger having catalyst-coated surfaces therein.
 14. In a process for cracking hydrocarbons, hydrocarbon mixtures or hydrocarbon fractions, the improvement wherein the reaction tubes of an existing cracking unit are coated with catalyst.
 15. In a cracking furnace for cracking hydrocarbons, hydrocarbon mixtures or hydrocarbon fractions in reaction tubes, the improvement wherein said reaction tubes are coated with a catalyst.
 16. In an ethylene production unit, for cracking hydrocarbons, hydrocarbon mixtures or hydrocarbon fractions in reaction tubes, the improvement wherein said reaction tubes are coated with a catalyst.
 17. In a steam reformer, for cracking hydrocarbons, hydrocarbon mixtures or hydrocarbon fractions in reaction tubes, the improvement wherein said reaction tubes are coated with a catalyst.
 18. In a cracking furnace, steam reformer, reactor, cracked gas cooler, quench exchanger, evaporator and/or heat exchanger, wherein surfaces that come into contact with carbon and/or carbon compounds during operation are coated at least partially with a catalyst to reduce or avoid coke deposits and to promote gasification of coke and/or cracking reactions.
 19. An apparatus according to claim 18, wherein the catalyst coating is applied directly on said surfaces or on an adhesion promoter layer previously applied to said surfaces and/or on an auxiliary medium placed adjacent said surfaces.
 20. An apparatus according to claim 19, wherein the catalyst coating is applied as a thermal coating, a cold coating, a vapor-deposited coating or an adhesive coating.
 21. An apparatus according to claim 19, wherein the catalyst coating contains calcium aluminate.
 22. A process according to claim 21, wherein said catalyst is doped with alkali vanadate.
 23. A process according to claim 22, wherein said alkali vanadate is potassium pyrovanadate. 